DFT-SOFDM is an OFDM-like single carrier modulation technique that is used in the EUTRA uplink (25.814 v2.0.0). DFT-SOFDM has significantly better power de-rating, which is also known as cubic metric or peak to average power ratio (PAPR) properties than OFDM, enabling better data rates near the cell edge and/or improved battery life in subscriber stations or user equipment (UE). Unfortunately, direct conversion transmitters and receivers introduce distortion on the DC sub-carrier. On the uplink, the distortion includes unsuppressed carrier feed-through from all active user equipment in the uplink.
In 3GPP 25.814 v2.0.0, the DC sub-carrier may be used for DFT-SOFDM transmissions. Since DFT-SOFDM is a weighted sum of multiple data symbols (in contrast to conventional OFDM), degradation to the DC sub-carrier degrades receiver performance compared to an ideal DFT-SOFDM receiver with no DC distortion. At the transmitter, both error vector magnitude (EVM) and the cubic metric (CM)/PAPR worsen with increased levels of DC distortion.
In 3GPP 25.814 v2.0.0, a DC sub-carrier is provided on the OFDM downlink (DL) but it is not used for data transmission. In IEEE 802.16, a DC sub-carrier is provided on the OFDMA uplink (UL) but it is not used for data transmission. Using this same concept on the DFT-SOFDM uplink would improve receiver performance, since no spread data is mapped to the DC sub-carrier, and help EVM. However, allocations spanning the DC sub-carrier would suffer from increased CM (˜1.7 dB for pi/2 BPSK, 0.7 dB for QPSK, and 0.5 dB for 16 QAM), and this would negate one of the benefits of DFT-SOFDM.
DFT-SOFDM has been proposed for the reverse link in 3 GPP2. 3GPP2 however does not discuss how to handle the DC sub-carrier with direct conversion transmitters and receivers in the system.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description and the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.